String calculator that has no precedence and is left-associative - c++

Question

The code for this string calculator has no operator precedence and I am not too sure how to fix it. I am confident it has something to do with the switch statement but I need this string calculator to move from left to right and calculate no matter if multiplication occurs before subtraction/addition or not. Here is the code so far:
#include<iostream>
#include<sstream>
#include<string>
#include<cctype>
#include<cmath>
using namespace std;
enum {PLUS='+',MINUS='-',MULT='*'};
int numberValue(string &expr)
{
istringstream is(expr);
int value = 0;
is >> value;
return value;
}
int expressionValue(string &expr)
{
int i = 0;
int p = 0;
if(expr.at(0) == '(' && expr.at(expr.length()-1) == ')')
{
for(i=0;i<expr.length();i++)
{
if(expr.at(i)=='(')
p++;
else if(expr.at(i)==')')
p--;
if(p==0)
break;
}
if(i==expr.length()-1)
return expressionValue(expr.substr(1,expr.length()-2));
}
for(i=0;i<expr.length();i++)
{
if(expr.at(i)=='(')
p++;
else if(expr.at(i)==')')
p--;
else if(p==0 && ispunct(expr.at(i)))
{
switch(expr.at(i))
{
case PLUS:
return expressionValue(expr.substr(0,i)) +
expressionValue(expr.substr(i+1,expr.length()-i- 1));
case MINUS:
return expressionValue(expr.substr(0,i)) -
expressionValue(expr.substr(i+1,expr.length()-i-1));
case MULT:
return expressionValue(expr.substr(0,i)) *
expressionValue(expr.substr(i+1,expr.length()-i-1));
}
}
}
return numberValue(expr);
}
bool Validate(string inputStr)
{
for(int x = 0;x < inputStr.length(); x ++)
{
if((inputStr[x] == '+') || (inputStr[x] == '-'))
{
if((inputStr[x+1] == '+') || (inputStr[x+1] =='-'))
{
return false;
}
}
}
string arr = "0123456789+-*";
int count = 0;
for(int a = 0; a < inputStr.length(); a++)
{
for(int b = 0; b < arr.length(); b++)
{
if(inputStr[a] == arr[b])
{
count++;
}
}
}
if(count == inputStr.length())
{
return true;
}
else
{
return false;
}
}
int main()
{
string expressionString;
string retry = "y";
cout << "Enter an expression as a string...you can use addition, subtraction \nand multiplication."<< endl;
cout << "\nKeep in mind that this calculator does not accept decimals and the \nleading operand can not be negative." << endl;
cout << "\nOperands can have no more than 8 digits, each operator has the same \nprecedence, and each operator is left associative." << endl;
do
{
cout << "\nEnter an expression: ";
cin >> expressionString;
if(Validate(expressionString) == true)
{
cout << expressionValue(expressionString) << endl;
}
else
{
cout << "An error has occured in the input" << endl;
}
cout << "Press \"y\" to enter another expression or \"n\" if you want to quit." << endl;
cin >> retry;
}while
(retry == "y");
return 0;
}

Answer 1

I've identified two problems with your code.
First, and easier to fix. Cin >> string will only take everything up to the first whitespace character. It might be worth considering http://www.cplusplus.com/reference/istream/istream/getline/ which will take everything up to the newline character. This will stop your program from just returning '1' if you type '1 + 1'
Your second problem, and the one I think you are more interested in, is the fact that you've accidentally made your operators right-associative. This is a consequence of the way your program is recursing. If you think about the way your function is calling itself for the statement 1 + 2 * 3. When it encounters the first + it calls ExpressionValue('1') + ExpressionValue(2 * 3). Because of this the multiplication will be evaluated before the addition in that case.
The easiest way to solve that problem will probably be to just parse the string backwards, but I suspect that will require changing the code that handles your parentheses quite a bit.
On a somewhat unrelated note. On the style side of things, some of your code was kind of hard to read because you don't indent the interiors of functions.